Production of metals



Oct. 24, 1961 E. s. cANDlDUs PRODUCTION oF METALS Filed April 24, 1958 Filament SUPPIY INVENTOR. Eck/a rq/ S CWM/:JW

3,005,859 PRODUCTICN F METALS Edward S. Candidns, Boston, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Apr. 24, 1958, Sex'. No. 730,575 2 Claims. (Cl. 13-31) This invention relates to melting of gas-containing metal particles of large surface area such as metal powders, sponge, scrap and the like,rand particularly to a furnace for melting s-uch gas-containing metal particles to produce gas-free metal ingots.

A principal object of the present invention is to provide a furnace for melting gas-containing metal particles to produce' gas-free metal ingots of extreme purity.

Another object of the invention is to provide a furnace ofthe above type which is simple to operate and is relatively cheap to construct.

Still another object of the invention is to provide a furnace of the above type which provides excellent control of the melting rate to insure complete degassing of the metal.

'Still another object of the invention is to provide a furnace of the above type which permits a relatively high rate of melting with the requisite high vacuum.

'These and other objects ofthe invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination or elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will beindicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following v detailed description taken in connection with the accompanying drawing which is a diagrammatic, schematic, sectional view of a preferred embodiment of the invention.

. In the present invention electron bombardment of the metal particles in a cold mold is employed for melting the metal under high Vacuum to produce a gas-free metal. In the degassification of metals, it is not essential that the absolute pressure be lower than about l to l0 microns Hg abs. It is desired that the metal be heated sufficiently to drive out dissolved and combined gases and volatilize metallic and other impurities. In the micron range of pressures the oxides and nitrides of many metals are either volatile or unstable at the melting point of the metal. Operating an arc-melting furnace at such pressures is extremely difficult unless the arc is between a consumable electrode and a bath of the metal being melted. The very nature oftheV consumable electrode melting operation, however, makes it extremely diiiicult to control the melting rate. As a general proposition, the melting operation, with a consumable electrode, is too rapid for complete degassiiication in a single melting operation. Accordingly, it hasbecome the practice in the industry to double or triple melt metals which are to be of ultimate purity.

While there have been suggestions in the art of small scale melting of metals by electron bombardment of the metal to be melted, this has not been very practical because of the fact that the source of electrons must be maintained at an extremely high vacuum, on the order of a few hundredths of a micron absolute pressure. This means that all of the gas liberated from the metal must be pumped out of the system at this extremely high vacuum. This requires large, expensive high vacuum diffusion pumps. Since the number of diffusion pumps that can be attached to a given furnace is limited by the size of the furnace, and since any diffusion pump has only a definite maximum capacity, the melting rate is limited to the available pumping capacity. 'Ihis has drastically limited the utility of electron melting to small scale laboratory equipment or to larger systems with expensive extremely high pumping capacities.

The present invention can best be understood by reference to the single ligure of the drawing which is a diagrammatic, schematic sectional view of one preferred embodiment of the invention. The furnace includes a vacuum tight chamber 10 having a cold mold 12 which permits maintenance of a small pool 14 of molten metal without contamination of the metal, since the interface between the mold and the metal is maintained solid. The furnace chamber is provided with a high capacity vacuum pump 16, such as a mechanical blower or diffusion pump which can handle large volumes of gases in the 1 to l0 micron pressure range. While this is not a high vacuum,l

the group V and group VI metals. At 18 there is sche matically indicated a supply of metal powder or sponge and at 19 there is shown a conveyer means for feeding metal powder into the molten pool in the cold mold. In one embodiment of the invention, the cold mold 12 is preferably designed with a retractable mold stool 20 which can `be gradually lowered by means of a feed screw 22 as the melting proceeds to permit the plane of the molten pool surface to remtain substantially constant.

A cathode 24 for emitting electrons is positioned` in a cathode chamber 26 communicating with the furnace chamber 10. This cathode 24 is preferably a concave tantalum or tungsten element and is arranged to be heated by a heater element 28. The cathode 24 has a focusing electrode 30 which, in combination with an accelerating anode 32 provides a high energy, focused beam 34 of` rected into the furnace chamber and onto the furnace of the molten pool in the cold mold. Each aperture is pref-- erably in the form of a cylinder which is coaxial with the electron beam and is slightly larger than the focused beam 34 of electrons to permit passage of the electron beam v therethrough but to inhibit, as much as possible, the flow of gas from the furnace chamber 10 to the cathode chamber 26.

In one embodiment of the invention illustrated, there are three such apertures and three pumps. The first pump 36 provides for the maintenance of a very high vacuum (eg. 1x10-t'5 mm. Hg abs.) in the cathodel chamber 26. The second pump 46 maintains a pressure of about 0.1 micron (l 10-4 mm. Hg abs.) in the space between the partitions 40 and 42, while the third pump 48 maintains a pressure of about l micron between the partitions 42 and 44. The furnace chamber 10 is connected to the booster pump 16 which will maintain the pressure in 10 between l and 10 microns.

In addition to the elements enumerated above, the fur nace includes a plurality of focusing coils for keeping the electron beam focused in a narrow beam for passage through the restricted openings in the partitions 40, 42 and '44. A suitable power supply 52 is provided for the focusing coils. A high voltage power supply 54 is connected to the elements of the electron emitter 24 and beam forming anode 32. Suitable water connectors 56 are shown for supplying cooling water from a source thereof (not shown) to cool the mold 12 and the mold Sto-0.12.0-

`The present invention permits the holding of the molten pool for any desired `tirne topermit purification of the melt. Thus, where the degassiica-tion kor volatilization of impurities is dependent upon a definite time-temperature relationship, it fis possible to achieve the desired timetemperature relationship lby melting at a sufficiently slow rate. The invention also has the advantage that it eliminates the possibility of dangerous shortfcircuits by gas discharge in the high voltage portion of the apparatus. This has the effect of greatly simplifying the design of the electrical system and lowers the cost of ,the highy voltage power supply. The present invention also permits use of rather delicate cathode surfaces having materials of high electron `emission without danger of'spoiling such surfaces due to a pressure burst inthe furnace. In this casel a ystandard high-vacuumfvalve is preferably p-laced in tube 38 Yto maintain'a high vacuum at all times in the vicinity of the cathode.

While one preferred embodiment of the invention has -been described above, Vnumerous modifications thereof can be made without departing from the scope of the invention. For example, more pumping chambers can be employed along the path of travel of the electron beam thereby decreasing the total volumetric capacity of thje requisite pumps but increasing the number yof such pumps. Similarly, the'number of chambers can be ,decreased with increasing volumetric pumping requirements. Numerous other heating means and electron beam forming systems can be employed as well as other standard methods for feeding particular metal pieces such as sponge, chips, scrap, powder and the like.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it isV intended that all matter contained in the above description orshown in the laccompanying drawings be interpreted as illustrative and not in a limiting sense. v 'r Y lWhat is claimed is:

1. A furnace for melting gas-containing metal to produce gas-,free metals, said furnace comprising a vacuumtight furnace chamber, 'a' 'cold mold in said chamber, means for evacuating said chamber to a free air pressure on the order gof l micron Hg abs., a cathode, means for heating said cathode to produce z,thermal emission of elec trons therefrom, means for accelerating said electrons and forming said electrons into a beam, said cathode being in a chamber which is isolated from said furnace chamber by at -lea st one wall vcontaining an aperture, means for evacuating said cathode chamber `to a lower pressure than the furnace chamber to provide a low pressure Zone, a magnetic -focusing means creating'a magnetic focusing iield Larranged to direct said electron beam from V said low pressure zone through said aperture to the higher pressure zone, said magnetic field providing direction of said beam towards said cold mold with a force which is essentially free of voltage gradient and thus preventing electrical discharges despite the existence of an electrical-discharge-supporting pressure with a high'degree of ion formation in the path ofthe 4beam adjacent said mold,

the distance between the nearest aperture through which the electron beam passes and the upper edge of the cold mold being substantially greater than the diameter of the cold mold to provide a large unobstructed volume for escape of gases from metal being melted in the cold mold, means for moving metal to be melted into position to be contacted by said electron beam within said cold mold, said vacuum pumping means yassociated with said cathode chamber having a suiiicient pumping capacity to maintain said cathode chamber at a pressure less than .1 micron Hg abs. despite gases leaking through said aperture into the cathode chamber.

2. A furnace for melting gas-containing metal to produce gas-free metals, said furnace comprising a vacuumtight furnace chamber, a cold -mold in Asaid chamber, means for evacuating said chamber to a free air pressure on the order of 1 micron Hg abs., a cathode, lmeans for heating said cathode to produce thermal emission of electrons therefrom, means for `accelerating said electrons and forming said electrons into a beam, said cathode -being in a chamber which is isolated from said furnace chamber by at least one wall containing an aperture, means for evacuating said cathode chamber to a lower pressure than the furnace chamber to provide a low pressure zone, a first magnetic focusing means creating a first magnetic focusing field arranged to direct said electron beaml from said low pressure zone through said aperture to a higher pressure Zone, a second magnetic focusing means positioned to create a second magnetic focusing field yon the higher pressure size of said' first aperture, said second magnetic iield providing direction of said beam through a second aperture in a second wall towards said cold mold with a force which is essentially free of voltage gradient and thus preventing electrical discharges despite the existence of an electrical-discharge-supporting pressure with a high degree of ion formation in the path of the beam adjacent said mold, the distance between the nearest aperture through which the electron beam passes and the upper edge of the cold mold being substantially greater than the diameter of the cold mold to provide a large unobstructed volume for mcape of lgases from metal being melted in the cold mold, means for moving metal to be melted into position to be contacted by said electron beam within said cold mold, saidv moving means being positioned between said secondwall and said cold mold, said vacuum pumping means associated with said cathode chamber having a suiiicient pumping capacity to maintain said cathode chamber at a pressure less than .1 micron Hg abs. despite gases leaking through said aperture -into the cathode chamber.

References Cited in the tile of this 4patent UNITED STATES vvPATENTS 848,600 Von Pirani Mar. '26, 1907 1,326,794 Sinding-Larsen Dec. ,30, 1919 2,423,729 Ruhle I July S, 1947 2,541,764 tI-Ierres et al. Feb'. 13, 1951 2,727,936 Boyer l Dec. 20, 1955 2,771,568 Steigerwald Nov.'20, 1956 2,778,926 Schneider vIan. Y22', 1957 2,793,282 Steigerwald May 21, 1957 2,818,461 Gruber et al Dec. 3l, 19,57 

1. A FURNACE FOR MELTING GAS-COTANING METAL TO PRODUCE GAS-FREE METALS, SAID FURNACE COMPRISING A VACUUMTIGHT FURNACE CHAMBER, A COLD MOLD IN SAID CHAMBER, MEANS FOR EVACUATING SAID CHAMBER TO A FREE AIR PRESSURE ON THE ORDER OF 1 MICRON HG ABS., A CATHODE, MEANS FOR HEATING SAID CATHODE TO PRODUCE THERMAL EMISSION OF ELECTRONS THEREFROM, MEANS FOR AXCELERATING SAID ELECTRONS AND FORMING SAID ELECTRONS INTO A BEAM, SAID CATHODE BEING IN A CHAMBER WHICH IS ISOLATED FROM SAID FURNACE CHAMBER BY AT LEAST ONE WALL CONTAINING AN APERTURE, MEANS FOR EVACUATING SAID CATHODE CHAMBER TO A LOWER PRESSURE THAN THE FURNACE CHAMBER TO PROVIDE A LOW PRESSURE ZONE, A FIELD ARRANGED TO DIRECT SAID ELECTRON BEAM FOR SAID LOW PRESSURE ZONE THROUGH SAID APERTURE TO THE HIGHER PRESSURE ZONE, SAID MAGNETIC FIELD PROVIDING DIRECTION OF SAID BEAM TOWARDS SAID COLD MOLD WITH A FORCE WHICH IS ESSENTIALLY FREE OF VOLTAGE GRADIENT AND THUS PREVENTING ELECTRICAL DISCHARGES DESPITE THE EXISTENCE OF AN ELEC- 